Nitric oxide (NO) is a major messenger molecule in the cardiovascular, immune and nervous systems. In the brain, NO is responsible for the glutamate-linked enhancement of 3', 5' cyclic guanosine monophosphate (cGMP) levels (S. R. Jaffrey and S. H. Snyder, Annu. Rev. Cell Dev. Biol. 11, 417, 1995) and may be involved in apoptosis (E. Bonofoco et al., Proc. Natl. Acad Sci. U.S.A. 92, 7162, 1995; J. B. Mannick et al., Cell 79, 1137, 1994), synaptogenesis (Jaffrey and Snyder, ibid.; T. Wang, Z. Xie, and B. Lu, Nature 374, 262, 1995) and neuronal development (Jaffrey and Snyder, ibid.).
Since NO cannot be stored in vesicles like other neurotransmitters, its release is regulated by the activity of the enzyme which makes it, NO synthase (NOS). Although a number of substances are known to regulate transcription of NOS, it is possible that regulation occurs at other levels as well. For example, several enzymes are influenced by physiologically associated proteins that serve as enzyme inhibitors. Examples include cyclin-dependent kinase inhibitors (A. Kamb, Trends Genet., 11, 136, 1995; S. J. Elledge and J. W. Harper, Curr. Opin. Cell Biol. 6, 847, 1994), the FoF1 ATPase inhibitor (J. E. Walker, Curr. Opin. Struct. Biol. 4, 912, 1994), and the ornithine decarboxylase inhibitor antizyme (J. S. Heller, W. F. Fong, E. S. Canellakis, Proc. Natl. Acad. Sci. U.S.A. 73, 1858, 1976). Such regulation of NOS by protein inhibitors was not known.
NO mediates glutamate neurotoxicity, which has been implicated in debilitating and lethal neurodegenerative disorders such as Alzheimer's and Huntington's diseases (D. W. Choi, J. Neurosci 10, 2493-2501; B. Meldrum and J. Garthwaite, Trends Pharmacol. Sci. 11, 379-387, 1990). Thus, there is a continuing need in the art of neurotransmitter regulation for methods of affecting the activity of neuronal NOS, so that one can manipulate NO levels when required for therapeutic effect in such disorders.